ADSL for Broadband Access
By Martin Jackson
ADSL is a modem technology that converts an ordinary phone line into a high-speed digital pipe for ultra-fast access to the Internet and corporate networks while also enabling real-time multimedia services. With downstream speeds as high as 8 Mbps, ADSL is nearly 300 times faster than 28.8K dial-up modems and 70 times faster than paired 128-Kbps ISDN.
Telecommunication carriers (telcos) throughout the world have delivered voice services for more than a century. During this period, they have refined their services to meet customer's evolving needs and to take advantage of new technologies. Today our planet is laced with nearly 700 million telephone lines, a number which is expected to increase to nearly 1 billion by the year 2000.
Our communications needs have become more sophisticated as computers have become more powerful and widespread. Today's networks need to deliver information and services far beyond urban centres and corporate campuses to scattered users in branch offices, telecommuters, customers and various business partners. All are asking for faster access to mission-critical information on corporate networks as well as the Internet.
What's more, the Digital Revolution has spawned a host of multimedia and web-based applications that have led people to expect more than just text when they log on to the Internet or corporate networks. While these new multimedia applications provide increasingly more effective ways of communicating, they are clogging low-speed data pipelines.
The roadblock on the Information Superhighway is the current bandwidth limitations of the public switched telephone network (PSTN). Although most of the developed world is criss-crossed with powerful long-distance fiber-optic links, a bottleneck remains in the so-called "last mile." This local loop, feeding from the telephone company's central offices to the customer premises, consists of copper twisted-pair phone lines: analog veterans in the new digital age.
ADSL to the Rescue
Asymmetric Digital Subscriber Line (ADSL) neatly overcomes a number of limitations within the existing telephone network. ADSL is a modem technology that converts an ordinary phone line into a high-speed digital pipe for ultra-fast access to the Internet and corporate networks while also enabling real-time multimedia services.
With downstream speeds as high as 8 Mbps, ADSL is nearly 300 times faster than 28.8K dial-up modems and 70 times faster than paired 128-Kbps ISDN. ADSL is ushering in a new era of multimegabit access, satisfying today's desire for speed while paving the way for tomorrow's future interactive multimedia applications. ADSL has a number of benefits:
Copper bandwidth: ADSL exploits the unused spectrum capacity in ordinary phone lines, employing advanced modulation techniques to provide bigger, faster digital pipes for high-speed remote access. Hence ADSL can transmit at rates of up to 1.5 Mbps for distances of 18,000 feet and up to 8 Mbps for distances up to 12,000 feet.
Copper reuse: The installation, maintenance and operation of copper pairs has been the primary business of telcos for a long time and hence is well understood. By reusing the copper pair, ADSL enables telcos to capitalise on their existing investments and experience. This enables service providers to deploy ADSL technology more quickly. What's more, ADSL services can be rolled out on demand, subscriber by subscriber, as demand dictates
PSTN alleviation: ADSL will relieve congestion on voice telephony system increasingly bogged down by the growth of the Internet and long data transmissions. ADSL will enable carriers to offload data traffic onto a separate packet or cell-switched overlay network.
POTS (Plain Old Telephone Service): Because ADSL works in harmony with existing POTS services, the same copper pair can be used for both telephone and ADSL service. Thus, a single ADSL line offers simultaneous channels for PCs, TVs and telephones.
Other Access Technologies
ADSL is one of several access technologies which have emerged to support the requirement for high-speed local access. Direct broadcast satellite (DBS), cable modems and ISDN have all been offered as solutions, and each has its distinct advantages and disadvantages. These technologies are not necessarily mutually exclusive and very well may coexist, depending on user requirements.
DBS: DBS will typically provide a shared simplex bandwidth of about 30 Mbps downstream. Therefore although it may be suitable for some push-only services, it isn't a full service solution. There are a number of DBS-based Internet solutions which use DBS to deliver the data to the premises, but then uses the PSTN for the upstream direction. This once again means more data transmissions over the PSTN and insufficient upstream bandwidth for many services. ADSL can provide up to 8 Mbps downstream and 1 Mbps upstream on an exclusive, non-shared, basis.
Cable modems: Cable modems are similar in many ways to DBS providing a shared 30 Mbps downstream bandwidth. In the upstream direction they typically share out a 1 Mbps bandwidth between 1000 homes which gives each home an effective upstream bandwidth of 1 Kbps, far lower than conventional modems. Another significant problem is that cable modem plant is not normally full duplex which means that in order to deploy cable modems the operators have to upgrade their existing infrastructure, an expensive proposition and a new facet of operations with which cable modem operators must deal. ADSL plays to copper's strengths by reusing the plant and not interfering with existing services ‚ a double dip.
ISDN: ISDN provides a relatively small increase in bandwidth to 128 Kbps, best case, for a significant increase in complexity. In the United States, ISDN deployment continues to be painfully slow. One reason for this is that early on its life relevant standards were not in place. The result is a very complicated installation. The ADSL Forum is providing a fast track to standardisation for ADSL; Service providers, computer equipment companies and all major ADSL vendors are members working together to ensure that standards are in place for interoperable products.
ATM over ADSL for Multiple Service Delivery
ADSL and Asynchronous Transfer Mode (ATM) together provide an ideal solution for multiple service delivery. The ADSL Forum earlier this year approved a technical recommendation for running ATM traffic over ADSL modem links. Technical Report TR-002 helps pave the way for service providers who are eager to employ ADSL technology to offer high-speed access to the Internet and corporate networks‚while adding the virtual circuits and quality-of-service guarantees of ATM.
Most carriers would like data traffic to leave the customer site as ATM running over ADSL modem links, get aggregated via mutliplexers at the central offices and then dropped onto the high-speed ATM switching fabric. However, some providers favor other popular packet network protocols, such as Frame Relay, Ethernet, IP and IPX. To speed this process, the ADSL Forum's Technical Committee is organized into several working groups that will develop specifications for protocols for IP access and premises networks, outline network architectures and migration strategies and address issues of network management and ADSL testing.
ADSL, as its name implies, provides different rates in the upstream (from the premises) and downstream (to the premises) directions. This is, of course, ideally suited for the Internet as users will typically read far more than they deliver. However, ADSL is suited to far more than just casual Web browsing; businesses can use ADSL /ATM for all of their communications needs.
For the purposes of this paper, we will focus on running ATM traffic over ADSL modems. Businesses have a variety of communications needs: voice services, data services between sites, Internet access and video services. By using ADSL/ATM, carriers can provide all of these services with the necessary guarantees of quality. ATM simplifies telco infrastructure by enabling them to provision and manage a single network. This in turn reduces the amount of equipment and infrastructure that they have to maintain with consequent advantages for the consumers of the services.
ADSL-based networks are well suited for carrying ATM traffic, thus future-proofing ADSL technology for decades to come.
The services that businesses require have very different requirements and ATM serves these well:
Voice services: ATM can provision a constant bit rate (CBR) service with a guaranteed delay; the user hears good quality voice, without satellite type delays, and the telco does not have to add extra equipment, such as echo suppressors, to their network.
Data between sites: ATM provides a switched connection technology with flow control. Switched connections mean that users can separate private and public traffic on the same physical connection with the certainty that the two won't be mixed up. With flow control the user's data is delivered reliably from end to end without retransmissions. The result is higher throughput and lower costs: site to site communication will use services tariffed on a usage basis.
Internet access: ATM is already used by many companies for Internet access.
Video services: A growing number of companies are using video conferencing and video distribution in their business. Video quality is crucial. To provide constant video quality without annoying delays and resulting jerkiness, the network needs to support a guaranteed but variable bit rate. ATM has a variable bit rate (VBR) service which is designed to support this.
There are a number of competitive technologies to ATM for any one of these services but only ATM has been designed, from its inception, to support them all.
ADSL Deployment
The United States and Canada have had the widest press coverage for their ADSL trials and deployment. Many of the Baby Bells have had successful trials and are deploying now for ADSL service rollouts in early 1998.
The Asia Pacific region also have been leaders in this area. After a number of successful trials, ADSL deployment has already started for service rollout in the last quarter of this year. Korea and Singapore, for example, are well advanced in their deployment of ADSL. Korea Telecom has begun a pilot for their ADSL service in Pusan, the second largest city in Korea. The ADSL speed for the pilot is currently 4 Mbps downstream and 128 Kbps upstream, but this will be upgraded for the commercial roll-out. The first use of the service is high-speed Internet access but a major player in this project is a broadcasting company, and a video-on-demand service will be rolled out in future phases.
The first phase provides ADSL services to several hundred homes and offices. By the end of 1997, there will be about 3,000 end-points; 300,000 lines are expected to be deployed by the end of 1998. Recently, the Korean Minister of Communications announced that by the year 2002 there will be 3.5 million xDSL subscribers in Korea, with this number increasing to 5 million by 2005.
Singapore Telecom probably has the most aggressive ADSL rollout in the region. At Asia Telecom in June this year, Singapore's Prime Minister Goh Chok Tong launched the Singapore ONE network, which uses ADSL to provide high speed access to a public ATM backbone. The service, called Magix, is now available to Singaporeans.
Through Magix, the user can access the latest movies and news in addition to normal Internet access. There is also on-line shopping, games, music as well as a video enabled chat-line. About 500 homes and offices were using the service initially, and SingTel expects to have about 10,000 subscribers by the end of 1997 and about 80,000 by the end of 1998.
The core backbone of the Singapore ONE network, called 1-Net, is ATM based which enables multiple services to be supported over this single network (see Figure 1). In the access loop, ATM is carried over ADSL to support multiple service delivery to the homes, schools and businesses subscribing to the Magix service. The ADSL speed at roll-out is 5.5 Mbps downstream and 168 Kbps upstream, and the plan is to upgrade this to 8 Mbps and 1 Mbps respectively in the near future.
Other trials and pilots in the Asia Pacific region are shown in Table 1. ADSL is promising to be the access technology of choice for broadband services in this region as it allows the local telcos to begin deploying high value services with minimal capital investment.
Conclusion
As Ray Smith, CEO of Bell Atlantic, said "ADSL is an interim technology ‚ for the next forty years." From the telco's point of view, the broadband service allows equipment reuse, doesn't affect their existing offerings and enables them to genuinely offer a full range of communications based services. It's ideally suited for both residential and business customers with their widely different needs. By leveraging the world's nearly 700 million phone lines, ADSL offers the most viable option for providing virtually ubiquitous high-speed remote access and interactive multimedia transmission.
Martin Jackson is Technical Director for ATM Ltd and was recently elected to the Board of Directors for the ADSL Forum which is comprised of nearly 300 companies representing all sectors of the world's computer and communications industries.
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